Biomining

"NBIAP News Report." U.S. Department of Agriculture (June 1994).

Although mining is one of humankind's oldest activities, the techniques used to extract minerals haven't changed substantially for centuries. Ores are dug from the earth, crushed, then minerals such as copper and gold are extracted by extreme heat or toxic chemicals. The environmental and health effects of traditional mining technologies have been deleterious.

In the past few years, the mining industry has been turning to a more efficient and environmentally salubrious method for extracting minerals from ores: microorganisms that leach them out. Using a bacterium such as Thiobacillus ferooxidans to leach copper from mine tailings has improved recovery rates and reduced operating costs. Moreover, it permits extraction from low grade ores - an important consideration in the face of the depletion of high grade ores.

Thiobacillus ferooxidans, which is naturally present in certain sulfur-containing materials, gets energy by oxidizing inorganic materials, such as copper sulfide minerals. This process releases acid and an oxidizing solution of ferric ions, which can wash out metals from crude ore. Poor quality copper ore, which is bound up in a sulfide matrix, is dumped outside a mine and treated with sulfuric acid to encourage the growth of T. ferooxidans. As the bacteria chew up the ore, copper is released and collected in solution. The sulfuric acid is recycled.

Currently 25% of all copper worldwide, worth more than $1 billion annually, is produced through bioprocessing. This ranks it as one of the most important applications of biotechnology today. Bioprocessing is also being used to economically extract gold from very low grade, sulfidic gold ores, once thought to be worthless.

To increase the efficiency of biomining, the search is on for bacterial strains that are better suited to large-scale operations. Bioprocessing releases a great deal of heat, and this can slow down or kill the bacteria currently being used. Researchers are turning to heat-loving thermophilic bacteria found in hot springs and around oceanic vents to solve this problem. These bacteria thrive in temperatures up to 100 degrees Celsius or higher and could function in a high temperature oxidative environment.

Another effort is underway to find - or genetically engineer - bacterial strains that can stand up to heavy metals such as mercury, cadmium, and arsenic, which poison microbes and slow the bioprocessing. Some microbes have enzymes that protect their basic activities from heavy metals or pump them out. If genes that protect microbes from heavy metals can be identified, resistant strains might be engineered. In any event, biomining is now at the top of mining technology, and future development of the technology appears promising.